U.S. patent number 10,591,596 [Application Number 15/592,690] was granted by the patent office on 2020-03-17 for doppler resolution improvement in low-duty cycle transmission.
This patent grant is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS LLC. The grantee listed for this patent is GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to Igal Bilik, Moshe Laifenfeld, Alexander Pokrass.
United States Patent |
10,591,596 |
Pokrass , et al. |
March 17, 2020 |
Doppler resolution improvement in low-duty cycle transmission
Abstract
A system and method for obtaining a Doppler frequency of a
target are disclosed. A receiver receives a first plurality of
samples of a first echo signal from the target and a second
plurality of samples of a second echo signal from the target. The
second plurality of samples is separated from the first plurality
of samples by a time period. A phase shift is determined for the
duration of the time period and the phase shift is applied to the
second plurality of samples. The first plurality of samples is
combined with the second plurality of samples to obtain combined
samples, and the Doppler frequency for the target is obtained from
the combined samples.
Inventors: |
Pokrass; Alexander (Bat Yam,
IL), Bilik; Igal (Rehovot, IL), Laifenfeld;
Moshe (Haifa, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
GM GLOBAL TECHNOLOGY OPERATIONS LLC |
Detroit |
MI |
US |
|
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS
LLC (Detroit, MI)
|
Family
ID: |
63962426 |
Appl.
No.: |
15/592,690 |
Filed: |
May 11, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180329054 A1 |
Nov 15, 2018 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S
13/343 (20130101); G01S 13/931 (20130101); G01S
13/26 (20130101); G01S 13/584 (20130101); G01S
7/354 (20130101); G01S 13/30 (20130101); G01S
7/2923 (20130101); G01S 2007/356 (20130101) |
Current International
Class: |
G01S
13/93 (20060101); G01S 7/292 (20060101); G01S
13/34 (20060101); G01S 13/26 (20060101); G01S
7/35 (20060101); G01S 13/58 (20060101); G01S
13/931 (20200101); G01S 13/30 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Windrich; Marcus E
Attorney, Agent or Firm: Cantor Colburn LLP
Claims
What is claimed is:
1. A method of obtaining a Doppler frequency of a target,
comprising: obtaining a first plurality of samples of a first echo
signal from the target; obtaining a second plurality of samples of
a second echo signal from the target, wherein the second plurality
of samples is separated from the first plurality of samples by a
time period having a duration; shifting the second plurality of
samples with respect to the first plurality of samples by the time
period to combine the first plurality of samples and the second
plurality of samples; determining a phase shift corresponding to a
duration of the time period for a frequency of the first echo
signal determined from the first plurality of samples; applying the
phase shift to the second plurality of samples to remove a
discontinuity between a phase of the last sample of the first
plurality of samples and a phase of the first sample of the second
plurality of samples; and obtaining the Doppler frequency for the
target from the combined samples.
2. The method of claim 1, wherein the first plurality of samples
includes k samples and the second plurality of samples includes k
samples and the combined samples includes 2k samples.
3. The method of claim 1, wherein combining the first plurality of
samples and the second plurality of samples further includes
removing the time period.
4. The method of claim 3, wherein applying the phase shift to the
second plurality of samples corrects for the effect of removing the
time period.
5. The method of claim 1, wherein combining the first plurality of
samples and the second plurality of samples further includes
concatenating the first plurality of samples and the second
plurality of samples.
6. The method of claim 1, further comprising performing an FFT on
the combined plurality of samples to obtain the Doppler
frequency.
7. The method of claim 1, further comprising determining a relative
velocity of the target from the Doppler frequency.
8. The method of claim 7, further comprising maneuvering a vehicle
with respect to the target based on the relative velocity.
9. The method of claim 1, wherein the first plurality of samples is
a reflection from the target of a plurality of chirp signals
transmitted during a first transmission frame from the target and
the second plurality of samples is a reflection from the target of
a plurality of chirp signals transmitted during a second
transmission frame.
10. A system for obtaining a Doppler frequency of a target,
comprising: a receiver for receiving a first plurality of samples
of a first echo signal from the target and a second plurality of
samples of a second echo signal from the target, wherein the second
plurality of samples is separated from the first plurality of
samples by a time period having a duration; and a processor
configured to: shift the second plurality of samples with respect
to the first plurality of samples by the time period to combine the
first plurality of samples and the second plurality of samples,
determine a phase shift corresponding to a duration of the time
period for a frequency of the first echo signal determined from the
first plurality of samples, apply the phase shift to the second
plurality of samples to remove a discontinuity between a phase of
the last sample of the first plurality of samples and a phase of
the first sample of the second plurality of samples, and obtain the
Doppler frequency for the target from the combined samples.
11. The system of claim 10, wherein the first plurality of samples
includes k samples and the second plurality of samples includes k
samples and the combined samples includes 2k samples.
12. The system of claim 10, wherein the processor is further
configured to combine the first plurality of samples and the second
plurality of samples to remove the time period.
13. The system of claim 12, wherein applying the phase shift to the
second plurality of samples corrects for the effect of removing the
time period.
14. The system of claim 10, wherein the processor is further
configured to combine the first plurality of samples and the second
plurality of samples by concatenating the first plurality of
samples and the second plurality of samples.
15. The system of claim 10, wherein the processor is further
configured to perform an FFT on the combined plurality of samples
to obtain the Doppler frequency.
16. The system of claim 10, wherein the processor is further
configured to determine a relative velocity of the target from the
Doppler frequency.
17. The system of claim 16, wherein the processor is further
configured to maneuver a vehicle with respect to the target based
on the relative velocity.
18. The system of claim 10, wherein the first plurality of samples
is a reflection from the target of a plurality of chirp signals
transmitted by a transmitter during a first transmission frame and
the second plurality of samples is a reflection from the target of
a plurality of chirp signals transmitted by the transmitter during
a second transmission frame.
Description
FIELD OF THE INVENTION
The subject invention relates to a system and method for
determining relative velocity using radar and, in particular, to
improving a resolution for Doppler frequencies obtained in radar
systems using low-duty cycle transmission rates.
BACKGROUND
Automobiles and other vehicles have come to employ safety systems
which include radar technologies for detecting a location of an
object or target with respect to the vehicle so that a driver or a
collision-avoidance device can react accordingly. A radar system
includes a transmitter for sending out a source signal and a
receiver for receiving an echo or reflection of the source signal
from the target. The reflected signal is sampled at a selected
sampling frequency and the sampled data points are entered into a
Fast Fourier Transform (FFT) in order to determine a Doppler
frequency for the returning signal. A relative velocity of the
target with respect to the vehicle is determined from the Doppler
frequency.
The radar system transmits a series of chirp pulses, resulting in a
series of echo signals. The chirp pulses are transmitted in groups
known as transmission frames, with each frame including a plurality
of chirp signals. In order to operate the radar system within an
operational temperature range, transmission frames are separated by
a down-time period having a selected duration to allowing cooling.
Due to the down-time separating transmission frames, the size of
the FFT that can be performed is limited to the number of echo
signals obtained from a single transmission frame. It is known,
however, that the more signals that are sampled, the greater the
resolution of the Doppler frequency. Accordingly, it is desirable
to provide a method for increasing the number of the samples that
can be provided to the FFT in order to improve Doppler
resolution.
SUMMARY OF THE INVENTION
In one exemplary embodiment of the invention, a method of obtaining
a Doppler frequency of a target is disclosed. The method includes:
obtaining a first plurality of samples of a first echo signal from
the target; obtaining a second plurality of samples of a second
echo signal from the target, wherein the second plurality of
samples is separated from the first plurality of samples by a time
period having a duration; determining a phase shift for the
duration of the time period; applying the phase shift to second
plurality of samples; combining the first plurality of and the
second plurality of samples; and obtaining the Doppler frequency
for the target from the combined samples.
In another exemplary embodiment of the invention, a system is
disclosed for obtaining a Doppler frequency of a target. The system
includes a receiver for receiving a first plurality of samples of a
first echo signal from the target and a second plurality of samples
of a second echo signal from the target, wherein the second
plurality of samples is separated from the first plurality of
samples by a time period having a duration; and a processor. The
processor is configured to: determine a phase shift for the
duration of the time period, apply the phase shift to second
plurality of samples, combine the first plurality of and the second
plurality of samples, and obtain the Doppler frequency for the
target from the combined samples.
The above features and advantages and other features and advantages
of the invention are readily apparent from the following detailed
description of the invention when taken in connection with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features, advantages and details appear, by way of example
only, in the following detailed description of embodiments, the
detailed description referring to the drawings in which:
FIG. 1 shows a vehicle that includes a radar system suitable for
determining relative velocity of an object or target with respect
to the vehicle;
FIG. 2 shows a time diagram illustrating transmission signals and
echo signals obtained by operation of a radar system in accordance
with one embodiment of the invention;
FIG. 3 shows a graph illustrating the amplitudes for samples
obtained over the duration of the frames of FIG. 2;
FIG. 4 shows a graph illustrating the phases for the samples of
FIG. 2;
FIG. 5 shows a graph illustrating the real part of samples for
signals obtained over the duration of the frames of FIG. 2;
FIG. 6 illustrates a method for improving a resolution for a
Doppler frequency of sampled signals in one embodiment of the
invention;
FIG. 7 shows a flowchart illustrating the method disclosed herein
for improving a resolution for a Doppler frequency of sampled
signals in one embodiment of the invention; and
FIG. 8 shows a graph of exemplary frequency curves obtained using
different Fast Fourier Transform sample sizes.
DESCRIPTION OF THE EMBODIMENTS
The following description is merely exemplary in nature and is not
intended to limit the present disclosure, its application or uses.
It should be understood that throughout the drawings, corresponding
reference numerals indicate like or corresponding parts and
features.
In accordance with an exemplary embodiment of the invention, FIG. 1
shows a vehicle 100, such as an automobile, that includes a radar
system 102 suitable for determining relative velocity of an object
or target 104 with respect to the vehicle 100. In the embodiment
shown in FIG. 1, the radar system 102 includes a transmitter 106
and a receiver 108. In alternate embodiments, the radar system 102
may be a MIMO (multi-input, multi-output) radar system that
includes an array of transmitters and an array of receivers. A
control unit 110 including a processor on-board the vehicle 100
controls and operates the transmitter 106 to generate a radio
frequency wave (a "source signal" 120). In one embodiment, the
source signal 120 includes a linear frequency-modulated continuous
wave (LFM-CW), often referred to as a chirp signal. Alternatively,
the source signal 120 can be a pulsed signal or a combination of
pulsed and chirp signals. In one embodiment, the transmitter 106
transmits a sequence of transmission frames separated by down-time
periods, with each transmission frame including a plurality of
chirp signals. A reflection of the source signal 120 from the
target 104 is referred to herein as an echo signal 122. The echo
signal 122 is received at the receiver 108, which generally
includes circuitry for sampling the echo signal 122. The control
unit 110 performs a Fast Fourier Transform (FFT) on the sampled
signal to obtain frequencies in a frequency space in order to
determine a frequency of the echo signal 122 and thus a Doppler
frequency for the target 104. The Doppler frequency is used to
estimate the relative velocity of the target 104 with respect to
the vehicle 100.
Knowledge of the relative velocity of the target 104 with respect
to the vehicle 100 is used to maneuver the vehicle 100 by, for
example, accelerating or decelerating the vehicle 100 or steering
the vehicle to avoid the target 104. In one embodiment, the control
unit 110 cooperates with a collision-avoidance system 112 to
control steering and acceleration/deceleration components to
perform necessary maneuvers at the vehicle 100 to avoid the target
104. In another embodiment, the control unit 110 provides a signal
to alert a driver of the vehicle 100 so that the driver can take
necessary actions to avoid the target 104.
While the radar system 102 is discussed herein as being on-board a
vehicle 100, the radar system 102 may also be part of an immobile
or stationary object in alternate embodiments. Similarly, the
target 104 can be a vehicle or moving object or it can be an
immobile or stationary object.
FIG. 2 shows a time diagram 200 illustrating transmission signals
and echo signals obtained by operation of radar system 102 in
accordance with one embodiment of the invention. The top row 202
shows a sequence of transmission frames separated by down-time
periods. First transmission frame 212, second transmission frame
214 and third transmission frame 216 are shown sequentially in time
and are separated by down-time periods 213 and 215. First
transmission frame 212 is separated from second transmission frame
214 by down-time period 213. Second transmission frame 214 is
separated from third transmission frame 216 by down-time period
215. While FIG. 2 shows three transmission frames for illustrative
purposes, a transmission of signals may include any number of
transmission frames in alternative embodiments.
Each of the transmission frames 212, 214 and 216 includes a
plurality of chirp signals. The second row 204 shows chirp signals
of the transmission frames 212, 214 and 216 in one embodiment of
the invention. First transmission frame 212 includes 32 chirp
signals (labelled S.sup.1.sub.1 through S.sup.1.sub.32). Second
transmission frame 214 includes 32 chirp signals (labelled
S.sup.2.sub.1 through S.sup.2.sub.32) and third transmission frame
216 includes 32 chirp signals (labelled S.sup.3.sub.1 through
S.sup.3.sub.32). While 32 chirp signals are shown within each
transmission frame for illustrative purposes, any number of chirp
signals (that are powers of 2) may occur within a transmission
frame. In general, the number of chirp signals is the same for each
transmission frame. Each chirp signal lasts for a time duration
indicated by t.sub.Chirp, and chirp signals within a transmission
frame follow each other substantially without a pause.
The bottom row 206 shows sampled signals generated as a result of
reflection of the chirp signals of the second row 204 from a
target, such as target 104 of FIG. 1. Three sample frames 222, 224
and 226 including sample signals are shown corresponding to the
transmission frames 212, 214 and 216 respectively. First sample
frame 222 is followed by second sample frame 224 after a down-time
period 223 having duration T.sub.pause. Second sample frame 224 is
followed by third sample frame 226 after a down-time period 225
having duration T.sub.pause. Down-time period 227 follows third
sample frame 226. Each sample signal in bottom row 206 is created
in response to a chirp signal in second row 204. Within a sample
frame, sample signals are separated in time by the duration of the
chirp signal, t.sub.Chirp.
FIG. 3 shows a graph 300 illustrating the amplitudes for samples
obtained over the duration of the frames of FIG. 2. First set of
amplitudes 302 represent the amplitudes of samples from first
sample frame 222. Similarly, a second set of amplitudes 304
represent the amplitudes of samples from second sample frame 224,
and a third set of amplitudes 306 represent the amplitudes of
samples from the third frame 226. The first set of amplitudes 302,
second set of amplitudes 304 and third set of amplitudes 306 are
all of about equal intensity. Amplitudes 303, 305 and 307 are zero
during the down-time periods 223, 225 and 227.
FIG. 4 shows a graph 400 illustrating the phases for the samples of
FIG. 2. First set of phases 402 represents the phases of samples
from the first sample frame 222. Similarly, a second set of phases
404 represents the phases of samples from the second sample frame
224, and a third set of phases 406 represents the phases of samples
from the third sample frame 226. The first set of phases 402 begins
at zero for the first sample and increases linearly for each
successive sample. The phases of the second set of phases 404 and
third set of phases 406 also change in the linear fashion described
with respect to the first set of phases 402. Phases 403, 405 and
407 are zero during the down-time periods 223, 225 and 227.
FIG. 5 shows a graph 500 illustrating the real part of samples for
signals obtained over the duration of the frames of FIG. 2.
Waveforms 502, 504 and 506 are shown for sample frames 222, 224 and
226, respectively. Waveform 502 can be determined from the first
set of amplitudes 302 and first set of phases 402. Similarly,
waveform 504 can be determined from the second set of amplitudes
304 and second set of phases 404, and waveform 506 can be
determined from the third set of amplitudes 306 and third set of
phases 406. Waveforms have been filled in for the down-time period
between frames based on interpolation.
FIG. 6 illustrates a method 600 for improving a resolution for a
Doppler frequency of sampled signals in one embodiment of the
invention. The method includes combining a first frame (e.g., Frame
1, 601) of samples and an adjacent second frame (e.g., Frame 2,
603) of samples, each frame having k samples, to create a combined
or concatenated frame 609 of samples having 2*k samples. An FFT 613
is then performed on the combined frame of samples, whereas the
size of the FFT is 2*k, to obtain a Doppler frequency (617). This
process can be repeated on subsequent frames, as shown with frames
605 and 607 being combined into a frame 611 of size 2*k and FFT 615
being performed on frame 611 to obtain Doppler frequency (619).
In order to combine the first frame 601 and second frame 603 to
form frame 609, a phase shift due to the down-time period 602
between the first frame 601 and second frame 603 is taken into
account and the down-time period 602 is removed from between first
frame 601 and second frame 603. The phase shift is generally
applied to samples of the second frame 603 so that when the first
frame 601 and the second frame 603 are combined, there is little or
no discontinuity in the phases between last sample of the first
frame 601 and first sample of the second frame 603.
FIG. 7 shows a flowchart 700 illustrating the method disclosed
herein for improving a resolution for a Doppler frequency of
sampled signals in one embodiment of the invention. Box 702
schematically shows a collection of M frames of samples, with each
frame having k samples therein.
Box 704 shows a calculation method for determining a phase shift
due to the down-time period 602. The duration of the down-time
period 602 is measured as an integral number N=N.sub.chirp of
chirps with each chirp signal having duration T=t.sub.Chirp. The
phase shift is calculated using Eq. (1) below:
Phase=exp(j2.pi.*Ftr*T*N) Eq, (1) wherein the frequency Ftr can be
determined by performing an FFT on a frame of samples having size
k, such as the first frame 601 of samples. The frequency Ftr
determined from the first frame 601 may be used as a good first
estimate for determining the phase shift. The frequency Ftr can be
recalculated at later steps. For example, Ftr can be determined
from the FFT 113 of combined frame (i.e., frame 609). In one
embodiment, frames 601 and 603 can be recombined using the value of
Ftr determined from FFT 113. Alternatively, the value of Ftr
determined from FFT 613 can be used when combining frames 605 and
607.
In Box 706, adjacent frames are combined. The samples from a first
frame (e.g., frame 601) are concatenated with the samples of its
subsequent frame (e.g., frame 603). In Box 708, an FFT of size 2*k
is performed on the combined frame. In Box 710, a frequency of the
sampled signals is determined. The Doppler frequency is subsequent
determined.
FIG. 8 shows a graph 800 of exemplary frequency curves obtained
using different FFT sample sizes. Curve 802 shows a frequency curve
that is obtained using a single frame that includes 32 samples. The
frequency peak at about 520 Hertz (Hz) is broad and the size of
side lobes of curve 802 is comparable to the size of the frequency
peak. Curve 804 shows a frequency curve obtained using the methods
disclosed herein of concatenating adjacent sample frames. The peak
at about 520 Hz is narrower than the peak of curve 802, thus
providing better resolution of frequency. Additionally, side lobes
of curve 804 are significantly smaller than the peak of curve 804
and therefore do not interfere with frequency determination for
curve 804.
The methods disclosed herein improve the ability of a radar system
to distinguish react to a target. The improved Doppler frequency
measurements provide a improved value of relative velocity, which
can be provided to the driver or collision avoidance system in
order for the driver or collision avoidance system to have improved
reaction in avoiding the target, thus increasing a safety of the
driver and vehicle.
While the invention has been described with reference to exemplary
embodiments, it will be understood by those skilled in the art that
various changes may be made and equivalents may be substituted for
elements thereof without departing from the scope of the invention.
In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from the essential scope thereof. Therefore, it is
intended that the invention not be limited to the particular
embodiments disclosed, but that the invention will include all
embodiments falling within the scope of the application.
* * * * *